Explicitly set conflicting requirement for attrs.

This fixes an issue that just emerged in Travis CI:

$ make coverage-test
fatal: No names found, cannot describe anything.
coverage run --rcfile=".coveragerc" -m twisted.trial ./bridgedb/test/test_*.py
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/twisted/trial/__main__.py", line 9, in <module>
    load_entry_point('Twisted', 'console_scripts', 'trial')()
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/pkg_resources/__init__.py", line 489, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2793, in load_entry_point
    return ep.load()
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2410, in load
    self.require(*args, **kwargs)
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2433, in require
    items = working_set.resolve(reqs, env, installer, extras=self.extras)
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (attrs 18.2.0 (/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages), Requirement.parse('attrs>=19.2.0'), {'Automat'})
Coverage.py warning: No data was collected. (no-data-collected)
Makefile:76: recipe for target 'coverage-test' failed
make: *** [coverage-test] Error 1
The command "make coverage-test" exited with 2.